An endemic area of sporotrichosis is described in the Lake of Ayarza District, South Guatemala, where 53 patients have been observed within 3 years. In 45.3% of the cases, the infection appeared after handling fish. The disease was more frequently observed in man (83%) and in patients less than 30 years of age. The most frequent clinical type was the ascending lymphocutaneous sporotrichosis of the limbs. Some cases of ulcerative or verrucous lesions were seen. Almost all the patients cured rapidly either by potassium iodide (46 patients) or spontaneously, or after application of local heat (4 patients). Intradermal tests were performed in healthy population in the endemic area and, for comparison, in Guatemala City. Whole yeast cell antigens of Sporothrix schenckii and Ceratocystis stenoceras were used in these tests. Skin tests to both antigens were more frequently positive in the endemic area; the highest frequency was obtained with the antigen of C. stenoceras. A serological study was performed in 26 patients. The fluorescent antibody staining technique was more sensitive than yeast cell and latex particles agglutinations. C. stenoceras was isolated from bark of some trees, especially Eucalyptus, in the environment, but not S. schenckii.